A microwave switched multiplexer and a mobile telecommunications device including such a multiplexer

ABSTRACT

A microwave switched multiplexer having a bandpass Δf between frequencies f1 and f2, Δf=f1−f2, the multiplexer comprising an input microwave resonant waveguide; an output microwave resonant waveguide; and, n transmission channels where n&gt;1, each transmission channel coupled to the input microwave resonant waveguide and the output microwave resonant waveguide, each transmission channel having a transmission bandpass at a center frequency within Δf, the center frequencies of the transmission channels being equally spaced apart in frequency by Δf/n; each transmission channel comprising (a) an input resonator coupled to the input microwave resonant waveguide; (b) an output resonator coupled to the output microwave resonant waveguide: (c) a center resonator coupled to the input and output resonators, the three resonators being coupled together in cascade; (d) a tuning mechanism connected to the center resonator and adapted to be switched between on and off states, in the on state the resonant frequency of the center resonator being the same as that of the input and output resonators and in the off state the resonant frequency of the center resonator being outside the bandpass Δf.

The present invention relates to a microwave switched multiplexer. Moreparticularly, but not exclusively, the present invention relates to amicrowave switched multiplexer comprising n (n>1) transmission channelscoupled at each end to input and output microwave resonant waveguides,each transmission channel comprising input output and center resonators,the admittances of the input microwave resonant waveguide, inputresonator, center resonator and output resonator being in the ratios asclaimed. The present invention also relates to a mobiletelecommunications device including such a microwave switchedmultiplexer.

Switched multiplexers are constructed from the parallel connection of Nchannel filters each of which can be switched on or off at the same timethus producing a frequency dependent device which can have 2^(N)different states. Such devices have been available for over 30 years butsuffer from the fact that the frequency selective filters had increasinggroup delay and loss at the cross over point between channels and henceif two adjacent channels were switched on the increased loss and groupdelay occurred at the center of the combined channels.

One method used to overcome this problem is to power divide betweenalternate channels and recombine at the output. However, this techniqueincreases the overall loss by 6 dB. Also the rapid change in group delayat the cross over frequency makes phase tracking of devices (a necessityin interferometer systems) extremely difficult. Typically 8 or morechannels are required.

Microwave switched multiplexers having a plurality of transmissionchannels each comprising resonators coupled together in cascade are alsoknown. Such multiplexers however suffer from high loss, in particulareffectively impedance matching to the multiplexer across the passband ofthe multiplexer has to date proved impossible, resulting in high loss.

The present invention seeks to overcome the problems of the prior art.

Accordingly, the present invention provides a microwave switchedmultiplexer having a bandpass Δf between frequencies f1 and f2,Δf=f1−f2, the multiplexer comprising an input microwave resonantwaveguide;

-   -   an output microwave resonant waveguide; and,    -   n transmission channels where n>1, each transmission channel        coupled to the input microwave resonant waveguide and the output        microwave resonant waveguide, each transmission channel having a        transmission bandpass at a center frequency within Δf, the        center frequencies of the transmission channels being equally        spaced apart in frequency by Δf/n;    -   each transmission channel comprising    -   (a) an input resonator coupled to the input microwave resonant        waveguide;    -   (b) an output resonator coupled to the output microwave resonant        waveguide;    -   (c) a center resonator coupled to the input and output        resonators, the three resonators being coupled together in        cascade;    -   (d) a tuning mechanism connected to the center resonator and        adapted to be switched between on and off states, in the on        state the resonant frequency of the center resonator being the        same as that of the input and output resonators and in the off        state the resonant frequency of the center resonator being        outside the bandpass Δf;    -   the admittances of the input microwave resonant waveguide, input        resonator, center resonator and output resonator being in the        ratio

y/n:π/2:4x/π:π/2

-   -   where y is in the range 0.5 to 1.1 and    -   where y is in the range 0.5 to 1.5    -   the admittances of the input, center and output resonators        having absolute values such that for every transmission channel        when the tuning mechanism of the transmission channel is in the        on state and the tuning mechanisms of all other transmission        channels are in the off state the width of the transmission        bandpass of the transmission channel is substantially Δf/n

The microwave switched multiplexer according to the invention has noadditional loss at the cross over frequencies from one transmissionchannel to the next. Further, the group delay remains flat (linearphase) through the cross over when adjacent channels are switched on.Further, one can impedance match to the microwave switched multiplexeracross the passband with almost no additional loss.

Preferably y is in the range 0.8 to 1.2, more preferably 0.9 to 1.1,more preferably 0.95 to 1.05.

Preferably x is in the range 0.95 to 1.05, more preferably 0.97 to 1.03,more preferably 0.99 to 1.01.

Preferably n is odd. Alternatively n is even.

Preferably the admittance of the output microwave resonant waveguide isequal to the admittance of the input microwave resonant waveguide.

Preferably each resonator comprises an electrically conducting resonantcavity comprising first and second end faces and a side wall extendingtherebetween, the resonator further comprising a dielectric memberarranged within the cavity and spaced apart from the end faces;

-   -   the distance between the first and second end faces defining the        length of the cavity, the width of the cavity in a plane normal        to the length being at least four times the length.

Preferably the dielectric member of the center resonator comprises anaperture extending therethrough, the tuning mechanism comprising a rodwhich can be inserted or removed from the aperture to switch the centerresonator between on and off states.

Alternatively each resonator comprises an electrically conducing cavitycomprising first and second end faces and a side wall extendingtherebetween, the resonator further comprising a resonator postextending from an end face part way to the opposite end face.

Preferably the tuning mechanism comprises an electrical switch extendingbetween the resonator post of the center resonator to the spaced apartend face.

Alternatively each resonator is an FBAR or SAW resonator.

Preferably the microwave switched multiplexer further comprises amicrowave source connected to the input microwave resonant waveguide,the microwave source being adapted to provide a microwave signal havinga frequency between f₁ and f₂.

In a further aspect of the invention there is provided a mobiletelecommunications device comprising at least one microwave switchedmultiplexer as claimed in any one of claims 1 to 12.

The present invention will now be described by way of example only andnot in any limitative sense with reference to the accompanying drawingsin which

FIG. 1 shows an equivalent circuit for Y_(o)(p) of a transmission line;

FIG. 2 shows an equivalent circuit for Y_(e)(p) of a transmission line;

FIG. 3 shows an equivalent circuit for a combination of Y_(o)(p) andY_(e)(p) of a transmission line;

FIG. 4a shows insertion loss and return loss for a 20 transmissionchannel multiplexer not according to the invention with all channelsswitched on;

FIG. 4b shows the insertion loss and group delay for the multiplexer ofFIG. 4 a;

FIG. 5a shows the insertion loss and return loss for the multiplexer ofFIG. 4a with one channel switched off;

FIG. 5b shows the group delay for the multiplexer of FIG. 4a with onechannel switched off;

FIG. 6a shows the insertion loss and return loss for the multiplexer ofFIG. 4a with one channel switched on;

FIG. 6b shows the group delay for the multiplexer of FIG. 4a with onechannel switched on;

FIG. 7a shows the insertion loss and return loss for the multiplexer ofFIG. 4a with three channels switched off;

FIG. 7b shows the group delay for the multiplexer of FIG. 4a with threechannels switched off;

FIG. 8a shows the insertion loss and return loss for the multiplexer ofFIG. 4a with three channels switched on:

FIG. 8b shows the group delay for the multiplexer of FIG. 4a with threechannels switched off;

FIG. 9a shows the insertion loss and return loss for the multiplexer ofFIG. 4a with two channels switched off;

FIG. 9b shows the group delay for the multiplexer of FIG. 4a with twochannels switched off;

FIG. 10a shows the insertion loss and return loss for the multiplexer ofFIG. 4a with two channels switched on;

FIG. 10b shows the group delay for the multiplexer of FIG. 4a with twochannels switched on;

FIG. 11a shows the insertion loss and return loss for the multiplexer ofFIG. 4a with four channels switched off;

FIG. 11b shows the group delay for the multiplexer of FIG. 4a with fourchannels switched off;

FIG. 12a shows the insertion loss and return loss for the multiplexer ofFIG. 4a with four channels switched on;

FIG. 12b shows the group delay for the multiplexer of FIG. 4a with fourchannels switched on;

FIG. 13a shows the insertion loss and return loss for the multiplexer ofFIG. 4a with all channels switched on and the resonators having finiteQ;

FIG. 13b shows the group delay for the multiplexer of FIG. 4a with allchannels switched on and the resonators having finite Q.

FIG. 14(a) shows insertion loss and return loss for a 20 channelmicrowave switched multiplexer according to the invention with allchannels switched on;

FIG. 14(b) shows the group delay for the multiplexer of FIG. 14(a);

FIGS. 15(a) and 15(b) are equivalent plots to FIGS. 14(a) and 14(b) withone channel turned off;

FIGS. 16(a) and 16(b) are equivalent plots to FIGS. 14(a) and 14(b) withthree channels turned on;

FIGS. 17(a) and 17(b) are equivalent plots to FIGS. 14(a) and 14(b) withall channels on but with the resonators having finite Q;

FIG. 18 shows an embodiment of a multiplexer according to the inventionin cross section:

FIG. 19 shows a transmission channel of a further embodiment of amultiplexer according to the invention; and,

FIG. 20 shows the bandpass of a single transmission channel within thebandpass and close to the edges.

The key aspect of the design of the microwave switched multiplexeraccording to the invention is that it is designed as a parallelconnection of resonant elements such that in the ‘all on’ state thedevice represents a known circuit with no loss (in the lossless case)and constant group delay (linear phase) at all frequencies. Such acircuit is equivalent to a length of transmission line matched to theload and source resistance (normalised to 10 hm) and defined by atransfer matrix—

$\lbrack T\rbrack = \begin{bmatrix}c & s \\s & c\end{bmatrix}$

Where c=cos h (πp) and s=sin h (πp), where p is the normalised complexfrequency variable.

The reflection coefficient S₁₁(p) and the transmission coefficientS₁₁(p) are defined in terms of even Y_(e)(p) and odd Y_(o)(p)admittances of the network as

${S_{11}(p)} = {\frac{1 - {{Y_{e}(p)}{Y_{o}(p)}}}{\left( {1 + {Y_{e}(p)}} \right)*\left( {1 + {Y_{o}(p)}} \right)}\mspace{14mu} {and}}$${S_{12}(p)} = \frac{{Y_{e}(p)} - {Y_{o}(p)}}{\left( {1 + {Y_{e}(p)}} \right)*\left( {1 + {Y_{o}(p)}} \right)}$

Where from the above one has

${Y_{e}(p)} = {\tanh\left( \frac{\pi \; p}{2} \right)}$${Y_{o}(p)} = {\coth\left( \frac{\pi \; p}{2} \right)}$

And hence S₁₁(p)=0 and S₁₂(p)=e^(−πρ)

Hence, the circuit is matched at all frequencies with a linear phase ofπω (p=jω).

To form the equivalent parallel connection of circuit elements a partialfraction expansion of Y_(e)(p) and Y_(o)(p) is performed—

Hence,

${Y_{o}(p)} = {{\coth\left( \frac{\pi \; p}{2} \right)} = {{\sum\limits_{r = {- \infty}}^{r = {+ \infty}}\frac{Ar}{p + {j\; 2r}}} = {\frac{2}{\pi}{\sum\limits_{r = {- \infty}}^{r = {+ \infty}}\frac{1}{p + {j\; 2r}}}}}}$

Thus, the equivalent circuit for Y_(o)(p) is as shown in FIG. 1

For Y_(e)(p),

${Y_{e}(p)} = {{\tanh (p)} = {\frac{2}{\pi}{\sum\limits_{r = {- \infty}}^{r = {+ \infty}}\frac{1}{p + {j\left( {{2r} - 1} \right)}}}}}$

This may be decomposed into the sum of two infinite series as

${Y_{e}(p)} = {{\frac{1}{\pi}{\sum\limits_{r = {- \infty}}^{r = {+ \infty}}\left\lbrack {\frac{1}{p + {j\left( {{2r} - 1} \right)}} + \frac{1}{p + {j\left( {{2r} + 1} \right)}}} \right\rbrack}} = {{\frac{2}{\pi}{\sum\limits_{r = {- \infty}}^{r = {+ \infty}}\left\lbrack \frac{p + {2{jr}}}{\left( {p + {2{jr}}} \right)^{2} + 1} \right\rbrack}} = {\frac{2}{\pi}{\sum\limits_{r = {- \infty}}^{r = {+ \infty}}\left\lbrack \frac{1}{\left( {p + {j\; 2r}} \right) + \frac{1}{\left( {p + {j\; 2r}} \right)}} \right\rbrack}}}}$

Thus, the equivalent circuit for Y_(e)(p) is as shown in FIG. 2.

Combining the even and odd mode networks gives the circuit as shown inFIG. 3. The circuit of FIG. 3 can be viewed as a multiplexer comprisingan infinite number of transmission channels equally spaced apart infrequency. Each transmission channel comprises three resonatorsconnected together in cascade, an input resonator coupled to an inputwaveguide for receiving the microwave signal, an output resonatorcoupled to an output waveguide and a center resonator connected betweenthe two. The resonators are connected in cascade. In each transmissionchannel the admittances of the resonators are in the ratio π/2:4/π:π/2(and in fact have these absolute values in this normalised model). Theadmittances of equivalent resonators in each channel are the same (ieall input resonators have the same admittance, all center resonatorshave the same admittance and all output resonators have the sameadmittance). The transmission channels are spaced equally apart infrequency. Whilst the transmission channels are shown physically spacedapart the input resonators are each connected to a common signal inputpoint. Similarly the output resonators are connected to a common signaloutput point.

This idealised multiplexer with an infinite number of transmissionchannels has the property that it is matched at all frequencies and hasa constant delay at all frequencies. To convert this multiplexer into aswitched multiplexer a tuning mechanism is connected to the centralresonator of each transmission channel which can be switched between‘on’ and ‘off’ configurations. When in the ‘on’ configuration theresonant frequency of the associated central resonator in a transmissionchannel is the same as that of the input and output resonators of thatchannel. When in the ‘off’ configuration the resonant frequency of theassociated central resonator is remote from that of the input and outputresonators of that channel such that the central resonator iseffectively shorted out.

Independent of which transmission channels are switched on or off (Atransmission channel is said to be on if the tuning mechanism of thecenter resonator of the transmission channel is in the on configurationand off if the tuning mechanism is in the off configuration) the oddmode admittance remains the same. For the even mode admittance howeverif the central resonator is shorted that component of the even modenetwork becomes the same as the equivalent component of the odd modenetwork.

For a multiplexer with a finite number of transmission channels theelements which make up the transmission channels are identical to theelements in the infinite network. There is an important differencehowever. If there are n channels the multiplexer has a bandpass of 2n infrequency, with each transmission channel having a transmission bandpassof 2. The tuning mechanism when in the off state tunes the resonantfrequency of the associated center resonator out of the bandpass of themultiplexer.

If the microwave switched multiplexer having a finite number, n, oftransmission channels has a bandpass Δf between f₁ and f₂ then theadmittances of the resonators will still have the same ratios. However,the absolute values of the admittances need to be scaled such that thetransmission bandpass of each transmission channel is Δf/n. Thefrequencies of the resonators are set such that the transmissionchannels are spaced equally apart by Δf/n so together covering themultiplexer bandpass Δf. Another way of saying this is that thetransmission bandpasses are substantially contiguous.

The insertion loss and return loss for such a multiplexer with alltransmission channels on is shown in FIG. 4(a). In FIG. 4(b) theinsertion loss and group delay are plotted with a constant delay of 100ns across most of the multiplexer bandpass. The switched microwavemultiplexer has 20 transmission channels. Each transmission channel hasa transmission band 10 MHz in width. The transmission channels areequally separated from each other to cover the multiplexer bandpassbetween 2.5 and 2.7 GHz. The multiplexer further comprises additionalresonant cavities connected as stubs at the common signal input andoutput points having resonant frequencies above and below themultiplexer band. The additional resonant cavities approximate to thebehaviour inband of the missing n+1 to infinity channels. Thismultiplexer is not according to the invention and is included forexplanatory purposes only.

To illustrate what happens to the behaviour of such a multiplexer whentransmission channels are switched off only channels near to the centerof the multiplexer bandpass are considered. With this simplification atheoretical response can be derived by still considering the multiplexerto have an infinite number of channels.

For one channel off—

$\begin{matrix}{{Y_{o}(p)} = {\coth\left( \frac{\pi \; p}{2} \right)}} \\{{Y_{e}(p)} = {{{\tanh\left( \frac{\pi \; p}{2} \right)} - {\frac{2}{\pi}\left\lbrack {\frac{p}{p^{2} + 1} - \frac{1}{p}} \right\rbrack}} = {{\tanh\left( \frac{\pi \; p}{2} \right)} + {\frac{2}{\pi}\left\lbrack \frac{1}{p\left( {p^{2} + 1} \right)} \right\rbrack}}}}\end{matrix}$

Which gives

$\frac{S_{11}(p)}{S_{12}(p)} = \frac{\frac{2}{\pi}{\cosh^{2}\left( \frac{\pi \; p}{2} \right)}}{{p\left( {p^{2} + 1} \right)} - {\frac{1}{\pi}{\sinh \left( {\pi \; p} \right)}}}$

Hence, S₁₁(p) is zero at p=+/−j and transmission loss (S₁₂(p)) has athird order zero at p=0. This response is shown in FIG. 5a with groupdelay in FIG. 5b . The passband begins at p=+/−j.

One channel on—

${Y_{o}(p)} = {\coth\left( \frac{\pi \; p}{2} \right)}$${Y_{e}(p)} = {{\coth\left( \frac{\pi \; p}{2} \right)} - \frac{2}{\pi \; {p\left( {p^{2} + 1} \right)}}}$

Hence,

$\frac{S_{11}(p)}{S_{12}(p)} = \frac{{\pi \; {p\left( {p^{2} + 1} \right)}} - {\sinh \left( {\pi \; p} \right)}}{2{\sinh^{2}\left( \frac{\pi \; p}{2} \right)}}$

Since the numerator has a third order zero at p=0 and the denominatorhas a second order zero at p=0 then S₁₁(0)=0. Also, S₁₁(+/−j)=0 andS₁₂(+/−2j)=0. This response is shown in FIG. 6(a). The group delay isshown in FIG. 6b . In the passband which is between P=+/−j the groupdelay is below 100 ns at the center of the band rising towards the bandedge. The phase response therefore possesses a cubic variation relativeto linear phase around the center frequency. The phase response andgroup delay are obtained from the fractions 1+Y_(o)(p) and 1+Y_(e)(p) inthe denominator of equation 3. The phase response for 1+Y_(o)(p) isalways linear phase (constant delay) for any switching state. However,for this case one should consider the phase of the factor

${1 + {{Ye}(p)}} = {1 + {\coth\left( \frac{\pi \; p}{2} \right)} - \frac{2}{\pi \; {p\left( {p^{2} + 1} \right)}}}$

For p=jw the phase is πw/2 for linear phase and hence the phase equalsthis value at w=0 and w=+/−1. Therefore the phase is an equidistantapproximation to linear phase over the full passband between w=+/−1.

Three channels off—

For three adjacent channels turned off—

$\mspace{20mu} {{Y_{o}(p)} = {\coth\left( \frac{\pi \; p}{2} \right)}}$${Y_{e}(p)} = {{{\tanh\left( \frac{\pi \; p}{2} \right)} + {\frac{2}{\pi}\left\lbrack {\frac{1}{p\left( {p^{2} + 1} \right)} + \frac{1}{\left( {p - {j\; 2}} \right)\left( {\left( {p - {j\; 2}} \right)^{2} + 1} \right)} + \frac{1}{\left( {p + {j\; 2}} \right)\left( {\left( {p + {j\; 2}} \right)^{2} + 1} \right)}} \right\rbrack}} = {{\tanh\left( \frac{\pi \; p}{2} \right)} + \frac{6\left( {p^{4} - {3p^{2}} + 12} \right)}{\pi \; {p\left( {p^{2} + 1} \right)}\left( {p^{2} + 4} \right)\left( {p^{2} + 9} \right)}}}$

From which

$\frac{S_{11}(p)}{S_{12}(p)} = \frac{{\cosh^{2}\left( \frac{\pi \; p}{2} \right)}\left( {p^{4} - {3p^{2}} + 12} \right)}{{\left( {p^{4} - {3p^{2}} + 12} \right){\sinh \left( {\pi \; p} \right)}} - {\frac{\pi}{3}{p\left( {p^{2} + 1} \right)}\left( {p^{2} + 4} \right)\left( {p^{2} + 9} \right)}}$

Hence S₁₁(p) is zero at p=+/j3 and close inspection shows that S₁₂(p)has a third order zero at p=0 and doubled ordered zeros at p=+/−j2. Thisis illustrated in FIG. 7a with the group delay in FIG. 7 b.

Three channels on—

For three adjacent channels turned on—

${Y_{o}(p)} = {{{\coth\left( \frac{\pi \; p}{2} \right)}\mspace{14mu} {And}\mspace{14mu} {Y_{e}(p)}} = {{\coth (p)} - \frac{6\left( {p^{4} - {3p^{2}} + 12} \right)}{\pi \; {p\left( {p^{2} + 1} \right)}\left( {p^{2} + 4} \right)\left( {p^{2} + 9} \right)}}}$

Which gives

$\frac{S_{11}(p)}{S_{12}(p)} = \frac{{\frac{\pi}{3}{p\left( {p^{2} + 1} \right)}\left( {p^{2} + 4} \right)\left( {p^{2} + 9} \right)} - {{\sinh \left( {\pi \; p} \right)}\left( {p^{4} - {3p^{2}} + 12} \right)}}{2{\sinh^{2}\left( \frac{\pi \; p}{2} \right)}\left( {p^{4} - {3p^{2}} + 12} \right)}$

Close inspection shows that S₁₁(p) has single ordered zeros at p=0,p=+/−j3 but double ordered zeros at p=+/−j. This response is shown inFIG. 8a . From Equation 15 it can be readily deduced that the phaseresponse is an equidistant approximation to linear phase and equal atp=0, +/j. +/−j2, +/−j3 hence, equidistant over the entire passband. Thecorresponding group delay is shown in FIG. 8 b.

To deal with the even degree cases (ie the multiplexer having an evennumber of transmission channels having transmission bands arrangedsymmetrically about the center of the multiplexer passband) it is easierto perform the transformation p→p+j which produces symmetrical responsesaround p=0. For example for the second degree case one has

Two channels off—

$\frac{S_{11}(p)}{S_{12}(p)} = \frac{2{\sinh^{2}\left( \frac{\pi \; p}{2} \right)}\left( {p^{2} - 2} \right)}{{{\pi/2}{p\left( {p^{2} + 1} \right)}\left( {p^{2} + 4} \right)} + {{\sinh \left( {\pi \; p} \right)}\left( {p^{2} - 2} \right)}}$

Where S₁₁(p) is zero at p=+/−j2 and S₁₂(p) has a single ordered zero atp=0 and double ordered zeros at p=+/−j as shown in FIGS. 9a and 9 b.

Two channels on—

$\frac{S_{11}(p)}{S_{12}(p)} = \frac{{{\pi/2}{p\left( {p^{2} + 1} \right)}\left( {p^{2} + 4} \right)} + {{\sinh \left( {\pi \; p} \right)}\left( {p^{2} - 2} \right)}}{{- 2}\left( {p^{2} - 2} \right){\cosh^{2}\left( \frac{\pi \; p}{2} \right)}}$

Hence S₄₁(p) has a third order zero at p=0 and zeros at p=+/−j as shownin FIG. 10a . The phase response of S₁₂(p) is an equidistantapproximation to linear phase at p=0, +/−j, +/−j2 and the correspondinggroup delay is shown in FIG. 10 b.

Plots are also shown in FIGS. 11a, 11b, 12a and 12b for four channelsoff and on.

If finite dissipation loss (ie finite Q) is introduced into theresonators of the transmission channels then due to the good return lossand approximately constant delay the overall loss in the respectivetransmission channel passbands will be very flat. This is illustrated inFIGS. 13a and 13 b.

Whilst the above described microwave switched multiplexer clearly hashighly desirable it is not possible to effectively impedance match tothe multiplexer across the bandpass so resulting in unacceptable loss.

The microwave switched multiplexer according to the invention is similarto that described above. As before the switched multiplexer comprises afinite number of transmission channels each comprising input, center andoutput resonators connected in cascade. The admittances and resonantfrequencies of these resonators are as previously described. However,the input resonators of each transmission channel are coupled to acommon input microwave resonant waveguide. The input microwave resonantwaveguide received a microwave signal from an input microwave signalline. In contrast to the switched multiplexer described with referenceto FIGS. 4(a) to 13(b) the input microwave signal line is not directlycoupled to the transmission channels but only through the microwaveresonant waveguide. Similarly, the output resonators of eachtransmission channel are coupled to a common output microwave resonantwaveguide. The signal from the switched multiplexer is received via anoutput microwave signal line inserted into the output microwave resonantwaveguide. These common input and output microwave resonant waveguidescompensate for all of the infinite number of transmission channels thatare in the permanently off (odd mode) state and which are absent fromthe multiplexer having a finite number of transmission channels. Theseinput and output microwave resonant waveguides have fixed admittancevalues which are a function of the number of active channels. The valuesof the admittances are obtained by ensuring that the mid band delay iscorrect and a match occurs at the band edge frequencies of the entireswitched multiplexer in the odd state.

In practice this requirement can be realised by ensuring that theadmittances of the input microwave resonant waveguide, input resonators,center resonators and output resonators are in the ratio

y/n:π/2:4x/π:π/2

Where y is in the range 0.5 to 1.5 and x is in the range 0.9 to 1.1.Preferably y is in the range 0.8 to 1.2, more preferably in the range0.9 to 1.1, more preferably 0.95 to 1.05. Preferably x is in the range0.95 to 1.05, more preferably 0.97 to 1.03, more preferably 0.99 to1.01.

Shown in FIG. 14(a) is the insertion loss and return loss for amicrowave switched multiplexer according to the invention with allchannels switched on. In FIG. 14(b) group delay is plotted showingconstant delay across most of the multiplexer bandpass. As describedabove the microwave switched multiplexer is similar to that employed forFIGS. 4(a) and 4(b) except it now includes the input and outputmicrowave resonant waveguides. In this particular example theadmittances are set such that x=1 and y=1.05.

FIGS. 15(a) and 15(b) are equivalent plots to FIGS. 14(a) and 14(b) butwith one transmission channel of the microwave switched multiplexerswitched off.

FIGS. 16(a) and 16(b) are equivalent plots to FIGS. 14(a) and 14(b) withthree transmission channels on.

FIGS. 17(a) and 17(b) are equivalent to FIGS. 14(a) and 14(b) with allchannels on except the resonators have finite Q. As before the behaviourof the microwave switched resonator is substantially independent of Qvalue.

As can be seen from FIGS. 14(a) to 17(b) inclusion of the input andoutput microwave resonant waveguides in the microwave switchedmultiplexer has little effect on the desirable properties of themicrowave switched multiplexer. The important difference however is thatit is now possible to match to the microwave switched multiplexeraccording to the invention across the microwave bandpass with negligibleloss.

Shown in FIG. 18 is a specific embodiment of a microwave switchedmultiplexer 1 according to the invention in cross section. The microwaveswitched multiplexer 1 has a bandpass Δf between frequencies f_(t) andf₂ with Δf=f₁−f₂. The microwave switched multiplexer 1 comprises aninput microwave resonant waveguide 2 and an output microwave resonantwaveguide 3. Coupled to the input and output microwave resonantwaveguides 2,3 are three transmission channels 4. Each transmissionchannel 4 comprises an input microwave resonator 5, an output microwaveresonator 6 and a center microwave resonator 7. Each microwave resonator5,6,7 comprises an electrically conducting resonator cavity 8 comprisingfirst and second end faces and a side wall 9 extending therebetween. Thecavities 8 are shown in plan view and so only the side wall 9 ofcircular cross section is shown. Arranged in each cavity 8 is a circulardielectric member 10. Each dielectric member 10 is spaced apart from thefirst and second end faces of its associated cavity 8. The length ofeach cavity 8 is defined by the distance between its end faces. Thewidth of each cavity 8 in a plane normal to the length is at least fourtimes the length.

Extending through the center of each dielectric member 10 is a smallaperture 11. Associated with each center resonator 7 is a tuningmechanism 12. The tuning mechanism 12 comprises a finger 13 which can beinserted into the aperture 11 of the dielectric member 10 of the centerresonator 7. When the finger 13 is in the aperture 11 the centerresonator 7 (and the associated transmission channel 4) is in the offconfiguration with the center resonator 7 effectively being shorted (ormore generally having a resonant frequency out of the passband of themultiplexer 1). When the finger 13 is not in the aperture 11 the centerresonator 7 and the associated transmission channel 4 is in the onconfiguration. In the on configuration all of the resonators 5,6,7 in atransmission channel 4 have the same resonant frequency.

The input resonator 5 is coupled to the input microwave resonantwaveguide 2 by means of an aperture 14 in the side wall 9 of its cavity8 as shown. The output resonator 6 is coupled to the output microwaveresonant waveguide 3 by means of an aperture 15 in the side wall 9 ofits cavity 8 as shown. Similarly, the center resonator 7 is coupled tothe input and output resonators 5,6 by means of apertures 16 in the sidewall 9 of its resonator cavity 8.

Each transmission channel 4 is defined by a transmission bandpass havinga center frequency. The center frequency of a transmission channel 4 isdetermined by the resonant frequency of the resonators 5,6,7 (in the onstate) in that transmission channel 4. The resonators 5,6,7 may betuneable (by known means such as displacement of the dielectric memberor a metal rod within the resonator cavity 8) or may have a fixedfrequency.

The center frequencies of the transmission channels 4 are equally spacedapart. Generally, if the bandpass of the multiplexer 1 is Δf and thereare n transmission channels 4 they are spaced apart in frequency byΔf/n.

Considering one transmission channel 4 again, as explained above theadmittances of the resonators 5,6,7 are designed to be substantially inthe ratio input:center:output π/2:4/π:π/2. Whilst this ratio ofadmittances is ideal in a practical device a more general range ofadmittance ratios will still provide an acceptable performance. Moregenerally the ratio of admittances is in the range π/2:x4/π:π/2 with xin the range 0.9 to 1.1, more preferably 0.95 to 1.05, more preferably0.97 to 1.03, more preferably 0.99 to 1.01, more preferably 1.

The absolute values of the admittances of the resonators 5,6,7 within atransmission channel 4 are set so that the transmission bandpass of thetransmission channel 4 is equal to the separation between the centerfrequencies of the transmission channels, or in other words that thetransmission bandpasses are substantially contiguous. The transmissionchannels 4 interact and so the transmission bandpass of a channel ismeasured with that transmission channel 4 in the on state and all othertransmission channels 4 in the off state.

In the particular embodiment shown in FIG. 18 all of the inputresonators 5 have the same admittance. All of the output resonators 6have the same admittance and all of the center resonators 7 have thesame admittance, so producing transmission channels 4 which all have thesame transmission bandpass, merely separated equally in frequency.

The input and output microwave resonant waveguides 2,3 have the sameadmittances. The admittances of the input microwave resonant waveguide2, input resonators 5, center resonators 7 and output resonators 6 arein the ratio

y/n:π/2:4x/π:π/2

as previously described.

Typically the microwave signal is provided to the input microwaveresonant waveguide 2 of the microwave switched multiplexer 1 by means ofan input signal line 17 inserted into the input microwave resonantwaveguide 2. The input signal line 17 is typically connected to amicrowave source (not shown). The microwave source can be a microwavetransmitter adapted to provide a microwave signal with a frequency inthe range f₁ to f₂. Alternatively the microwave source may be an antennaadapted to receive a microwave signal in this range. The output from themicrowave switched multiplexer 1 is received from an output signal line18 which is inserted into the output microwave resonant waveguide 3.

Shown in FIG. 19 is one transmission channel 4 of an alternativeembodiment of a switched microwave multiplexer 1 according to theinvention. As before the transmission channel 4 comprises an inputresonator 5 coupled to an input microwave resonant waveguide 2, anoutput microwave resonator 6 coupled to an output microwave resonantwaveguide 3 and a center resonator 7 connected between the two. Theresonators 5,6,7 are connected in cascade.

In this embodiment each resonator 5,6,7 comprises an electricallyconducting resonator cavity 20 comprising first and second end faces21,22 and a side wall 23 extending therebetween. Arranged within thecavity 20 is a resonator post 24 extending from one end face 21 part waytowards the other end face 22. The resonators 5,6,7 are coupled to eachother and to the microwave resonant waveguides 2,3 through apertures 25in the side walls of their resonator cavities 20 as shown.

Connected to the center resonator 7 is a tuning mechanism 26. The tuningmechanism 26 comprises a switch 27 connected between the end of theresonator post 24 and the remote end face 22 of the resonator cavity 20.In the on configuration the switch 27 is open circuit and so the centerresonator 7 resonates at the same frequency as the input and outputresonators 5,6. In the closed configuration the switch 27 is closedcircuit so shorting the resonator post 24 and resonant cavity 20together.

In an alternative embodiment of the invention the microwave resonators5,6,7 of the transmission channels 4 are FBAR (thin film bulk acousticresonators) or alternatively SAW (surface acoustic wave) resonators.Such resonators 5,6,7 are known in the art and accordingly are notdescribed in detail. Such resonators 5,6,7 are more compact than theresonators described above. By employing such resonators 5,6,7 themicrowave switched multiplexer 1 can be made sufficiently compact thatit can be employed in mobile telecommunications devices such as mobiletelephones.

As mentioned above the behaviour of the microwave switched multiplexer 1according to the invention is substantially insensitive to the Q valuesof the resonators 5,6.7. Nonetheless, resonators having Q values in therange 500 to 4000, more preferably 1000 to 3000 are typically employed.

The microwave switched multiplexer 1 according to the inventionpossesses a number of unique properties—

For any transmission channel 4 in the switched on state the phaseresponse over the entire passband is an equidistant approximation tolinear phase and hence produces a constant delay over most of the band.

For any transmission channel 4 on the on state the passband has themaximum number of zeros in the return loss including both band edgefrequencies.

If two or more adjacent transmission channels 4 are switched on a singlebandpass filter response is created which has an equidistantapproximation to linear phase over the entire combined passband.Furthermore, the overall return loss function has the maximum number ofzeros over the entire combined filter response.

If finite dissipation loss is introduced into the resonators 5,6,7, inthe passbands, due to the good return loss and approximately constantdelay the overall loss in the respective passbands will be very flat.

Finally, it is possible to match to the microwave switched multiplexer 1across the transmission band with very little loss.

In the above description reference is made to the width of thetransmission bandpass of a single transmission channel. As thetransmission channels interact the width of the transmission bandpass ofa transmission channel is measured with only one transmission channelswitched on and all other transmission channels switched off. FIG. 20shows, in schematic form, insertion loss and return loss for a singletransmission channel within and close to the edges of a transmissionbandpass. As can be seen, at the center of the bandpass there is aninfinity in the return loss. At each edge of the bandpass there is afurther infinity in the return loss. The width of the transmissionbandpass is the difference in frequency between these two edgeinfinities.

1. A microwave switched multiplexer having a bandpass Δf betweenfrequencies f₁ and f₂, Δf=f₁-f₂, the multiplexer comprising: n signalchannels, where n>1, each signal channel having a signal bandpass at acenter frequency within Δf; the center frequencies of the signalchannels being equally spaced apart by Δf/n; each signal channelcomprising: (a) a switch having first, second and third ports, theswitch being adapted to be switched between a transmit position in whichthe first port is connected to the second port, a receive position inwhich the first port is connected to the third port and an off positionin which the first port is not connected to either second or thirdports; (b) a common line extending from an antenna end to the firstport, the common line comprising an input resonator and a centerresonator connected together in cascade, the center resonator beingcoupled between the input resonator and the first port; (c) a transmitline extending between the second port and a transmit end, the transmitline comprising an output resonator, the output resonator beingconnected in cascade with the input and center resonators when theswitch is in the transmit position; (d) a receive line extending betweenthe third port and a receive end, the receive line comprising an outputresonator, the output resonator being connected in cascade with theinput and center resonators when the switch is in the receive position;the admittances of the two output resonators being equal; the admittanceof the input resonator, center resonator and output resonator being inthe ratioπ/2:4x/π:π/2 where x is in the range 0.9 to 1; wherein for at least twosignal channels adjacent in frequency the antenna ends are connected toa common antenna node and at least one of (a) the receive ends areconnected to a common receive node or (b) the transmit ends areconnected to a common transmit node; and the admittances of the input,center and output resonators having absolute values such that for everysignal channel when the switch for that channel is in the transmit orreceive position and the switches for all other channels are in the offposition, the width of the signal bandpass of the signal channel issubstantially Δf/n.
 2. A microwave switched multiplexer as claimed inclaim 1, wherein for the at least two signal channels the receive endsare connected to a common receive node and the transmit ends areconnected to a common transmit node.
 3. A microwave switched multiplexeras claimed in claim 1, wherein for every signal channel the antenna endsare connected to a common antenna node, the transmit ends are connectedto a common transmit node and the receive ends are connected to a commonreceive node.
 4. A microwave switched multiplexer as claimed in claim 1,wherein x is in the range 0.95 to 1.05.
 5. A microwave switchedmultiplexer as claimed in claim 1, wherein at least one of input, centerand output resonators is an FBAR or SAW resonator.
 6. A microwaveswitched multiplexer as claimed in claim 1, wherein the antenna node isconnected to antenna.
 7. A microwave switched multiplexer as claimed inclaim 1, wherein the antenna node comprises an antenna resonator.
 8. Amicrowave switched multiplexer as claimed in claim 7, wherein the ratioof the admittance of the antenna resonator to the input resonator isy/n:π/2 where y is in the range 0.5 to 1.5.
 9. A microwave switchedmultiplexer as claimed in claim 8, wherein y is in the range 0.8 to 1.2.10. A microwave switched multiplexer as claimed in claim 1, wherein thetransmit node is connected to a transmitter, the transmitter beingadapted to provide a microwave signal between frequencies f₁ and f₂. 11.A microwave switched multiplexer as claimed in claim 1, wherein thetransmit node comprises a transmitter resonator, the ratio of theadmittances of the transmitter resonator to the output resonator beingy/n:π/2 where y is in the range 0.5 to 1.5.
 12. A microwave switchedmultiplexer as claimed in claim 1, wherein the receive node comprises areceiver resonator, the ratio of the admittance of the receiverresonator to the output resonator beingy/n:π/2 where y is in the range 0.5 to 1.5.
 13. A microwave switchedmultiplexer as claimed in claim 1, further comprising a controllerconnected to the switches for switching the switches between states. 14.A mobile telecommunications device comprising: at least one microwaveswitched multiplexer having a bandpass Δf between frequencies f₁ and f₂,Δf=f₁−f₂, the multiplexer comprising: n signal channels, where n>1, eachsignal channel having a signal bandpass at a center frequency within Δf;the center frequencies of the signal channels being equally spaced apartby Δf/n; each signal channel comprising: (a) a switch having first,second and third ports, the switch being adapted to be switched betweena transmit position in which the first port is connected to the secondport, a receive position in which the first port is connected to thethird port and an off position in which the first port is not connectedto either second or third ports; (b) a common line extending from anantenna end to the first port, the common line comprising an inputresonator and a center resonator connected together in cascade, thecenter resonator being coupled between the input resonator and the firstport; (c) a transmit line extending between the second port and atransmit end, the transmit line comprising an output resonator, theoutput resonator being connected in cascade with the input and centerresonators when the switch is in the transmit position; (d) a receiveline extending between the third port and a receive end, the receiveline comprising an output resonator, the output resonator beingconnected in cascade with the input and center resonators when theswitch is in the receive position; the admittances of the two outputresonators being equal; the admittance of the input resonator, centerresonator and output resonator being in the ratioπ/2:4x/π:π/2 where x is in the range 0.9 to 1; wherein for at least twosignal channels adjacent in frequency the antenna ends are connected toa common antenna node and at least one of (a) the receive ends areconnected to a common receive node or (b) the transmit ends areconnected to a common transmit node; and the admittances of the input,center and output resonators having absolute values such that for everysignal channel when the switch for that channel is in the transmit orreceive position and the switches for all other channels are in the offposition, the width of the signal bandpass of the signal channel issubstantially Δf/n.
 15. (canceled)
 16. (canceled)
 17. A microwaveswitched multiplexer as claimed in claim 1, wherein x is in the range0.97 to 1.03.
 18. A microwave switched multiplexer as claimed in claim8, wherein y is in the range 0.9 to 1.1.
 19. A microwave switchedmultiplexer as claimed in claim 11, wherein y is in the range 0.8 to1.2.
 20. A microwave switched multiplexer as claimed in claim 11,wherein y is in the range 0.9 to 1.1.
 21. A microwave switchedmultiplexer as claimed in claim 12, wherein y is in the range 0.5 to1.2.
 22. A microwave switched multiplexer as claimed in claim 12,wherein y is in the range 0.9 to 1.1.